The invention relates to an antenna system, especially for an anti-theft system of a motor vehicle. Such antenna system are used for the purpose of the wireless reception of coded information from a code transmitter and forwarding it to a control unit to check it for entitlement.
A known antenna system (DE 195 42 441 A1) has several coils as transmitting and receiving antennas which are distributed within the motor vehicle. A query signal is emitted through the sending and receiving antenna and is received by a portable transponder. The transponder then responds with an answering signal in which coded data is contained. The coded data is evaluated in a control unit in which it is compared with expected set data. If the data agree, a releasing signal is produced by which the door lock is locked or unlocked or an immobilizer of the vehicle is unlocked.
The signals are transmitted inductively by means of a magnetic field. Such magnetic fields, however, have a limited range, so that not every corner within the vehicle and directly around the vehicle can be pervaded by the magnetic field with sufficiently great field strength. Areas in which the magnetic field strength is too low are referred to as null areas. If the portable transponder finds itself in such a null area it receives no query signal from the sending and receiving antenna. Since the signals emitted by the transponder in turn have only a very short range, no response signal can be received from the sending and receiving antenna.
Furthermore, a merchandise theft apparatus is known (EP 0 414 628 A2) which has a plurality of antenna coils connected in parallel and lying in one plane, and a signal is emitted by each. Parallel to the antenna coils the additional antenna coils of a receiver are arranged, by which a signal from a code sender is received. The magnetic field through which a request signal is transmitted to the code sender is produced only by the antenna coils. Transformers at the transmitter output serve only to match the impedance of the transmitter to the antenna coils.
This problem is solved according to the invention by the features of claim 1. An antenna system has a magnetic coupling means which is connected by a conductor to a secondary coil. The magnetic coupling means is magnetically coupled to a sending and receiving antenna and carries signals from the sending and receiving antenna to the secondary coil or vice versa. The coupling means and the secondary coil do not have to be connected galvanically to the sending and receiving antenna. Thus the latter can be routed regardless of any conductors present or of a wiring harness. Also, such coupling means with connected secondary coil can be easily placed at nearly any location.
Advantageous embodiments of the invention are specified in the subordinate claims. Thus a plurality of secondary coils can be connected to a single coupling means. The secondary coils can thus be spaced closely together in a limited area. The secondary coils can be arranged in the driver""s door, in the passenger""s door, in the vehicle roof, in the vehicle floor, in the trunk lid, in the mudguard, in the car seats, in the dashboard, etc. Since the secondary coils can be made relatively small, they are largely independent of their placement. If the conductor between the coupling means and the secondary coil is a twisted pair, the power losses of the signals sent through the conductor are very low.
The invention is addressed to the problem of creating an antenna system for an anti-theft system whereby it will be possiblexe2x80x94largely independently of the location of the code transmitterxe2x80x94for signals to be securely sent to the code transmitter and received by the latter as soon as the code transmitter is situated in the vicinity of the antenna system.
This problem is solved according to the invention by the features of claim 1. An antenna system has a magnetic coupling means which is connected by a conductor to a secondary coil. The magnetic coupling means is magnetically coupled to a sending and receiving antenna and carries signals from the sending and receiving antenna to the secondary coil or vice versa. The coupling means and the secondary coil do not have to be connected galvanically to the sending and receiving antenna. Thus the latter can be routed regardless of any conductors present or of a wiring harness. Also, such coupling means with connected secondary coil can be easily placed at nearly any location.
Advantageous embodiments of the invention are specified in the subordinate claims. Thus a plurality of secondary coils can be connected to a single coupling means. The secondary coils can thus be spaced closely together in a limited area. The secondary coils can be arranged in the driver""s door, in the passenger""s door, in the vehicle roof, in the vehicle floor, in the trunk lid, in the mudguard, in the car seats, in the dashboard, etc. Since the secondary coils can be made relatively small, they are largely independent of their placement. If the conductor between the coupling means and the secondary coil is a twisted pair, the power losses of the signals sent through the conductor are very low.